On 22 Feb 2023, at 23:01, Matthew Wilcox wrote: > On Wed, Feb 22, 2023 at 04:55:20PM -0700, Yu Zhao wrote: >> Currently anonymous folios only support two orders: 0 and the PMD >> order. Flexible orders for anonymous folios can resist both internal >> or external fragmentations when the PMD order is too underutilized or >> costly to allocate. Flexible orders can also leverage the TLB >> coalescing feature, .e.g., order 3 for AMD and order 4 for ARM CPUs. >> >> Discussion points: >> 1. The page fault path: determining the best order and the fallback policy. >> 2. The reclaim path: detecting the utilization and the splitting policy. >> 3. The refcount and the mapcount models, e.g., reuse the PMD-mapped >> THP model or not. >> 4. The splitting, and the collapsing if needed. >> 5. Other paths: COW, GUP, madvise(), mprotect(), page migration, etc. > > 6. Swap out an entire folio instead of splitting it before swap. > 7. At some point we're going to want Zi Yan's patches to split a folio > into arbitrary order folios instead of just to order-0. Since you mentioned it, I rebased it here: https://github.com/x-y-z/linux-1gb-thp/tree/split_thp_to_any_order_mm-everything-2023-03-04-00-19 . I will send it out whenever it is needed. :) > 8. For file folios, folio->index % folio->nr_pages is 0. Do we want > to maintain that invariant for anon folios? It helps tile the > folios so we don't end up with say, order-0, order-4, order-2 folios > in a sequence. > 9. How do we ensure that two page faults do not overwrite each others > folios, eg PF1 decides to allocate an order-0 folio at index 6 and > PF2 decides to allocate an order-2 folio at index 4? > > Probably some other things left to decide. > >> Now it seems to be a good time to revisit. Yang Shi and I have been >> looking at some details, trying to scope out the work that would be >> required. We don't have anything definitive yet, but we should have >> enough to share by the time of the LSF/MM/BPF. > > Excellent. There are other people also interested in this. eg: > > Ryan Roberts: https://lore.kernel.org/linux-mm/4c991dcb-c5bb-86bb-5a29-05df24429607@xxxxxxx/ > Fengwei Yin: https://lore.kernel.org/linux-mm/20230109072232.2398464-1-fengwei.yin@xxxxxxxxx/ > > Obviously I'm keenly interested. > https://lore.kernel.org/linux-mm/Y%2FU8bQd15aUO97vS@xxxxxxxxxxxxxxxxxxxx/ > > This is definitely worth its own session in the MM track. -- Best Regards, Yan, Zi
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